1. Field of the Invention
The present invention relates to a coordinate detection device mounted in an information terminal apparatus, such as cellular phones or PDAs. More particularly, the present invention relates to a capacitive coordinate detection device in which a plurality of X electrodes and a plurality of Y electrodes are arranged to cross each other at both sides of a base sheet.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-123363 described below is an example of the related art.
In Japanese Unexamined Patent Application Publication No. 2002-123363, a switch element 11 in which a reversion plate 10 is disposed is provided at a lower side, and a capacitive coordinate detection device (a flat panel input device 4) is provided at an upper side. In the switch element 11, an electrode 13a1 disposed at the center of the switch element 11 and a ring-shaped electrode 13a2 disposed near the switch element 11 are formed on a resin sheet 12. Diaphragm type reversion plates 15 made of metal are disposed on a surface of the ring-shaped electrode 13a2. Meanwhile, the flat panel input device 4 includes a flexible resin sheet. X-direction detection electrodes and Y-direction detection electrodes are disposed opposite to each other on top and bottom surfaces of the resin sheet in a matrix. On the surface of the resin sheet is laminated a display sheet 7 in which characters, symbols, figures, or the like are printed.
If a finger that touches the display sheet 7 is strongly pressed, the reversion plates 15 are reversed. As the electrode 13a1 located at the center of the switch element 11 and the ring-shaped electrode 13a2 become conductive, the input as a switch is made possible. Furthermore, since the reversion plates 15 are reversed by operating the switch, it is possible to give a feeling of clicking to an operator.
In the above-described device, if characters are written while moving a finger that softly touches the surface of the display sheet 7, the flat panel input device 4 can detect a motion trace of the finger as positional data every time on an XY coordinate axis based on variations in a capacitance between the X electrodes and the Y electrodes. It is thus possible to input the characters.
An example of the capacitive coordinate detection device can include Japanese Unexamined Patent Application Publication No. 08-137607.
In the above-mentioned information terminal device, it is necessary to let an operator know the position of the switch (the positions of characters, etc.) even in the dark. Therefore, it is preferable that a light source such as a LED for brightly displaying characters, symbols, figures or the like, which are printed on surfaces of switch buttons be disposed within the device, and the rear surface of the display sheet 7 be illuminated from the inside of the device.
However, the flat panel input device 4 is disposed between the light source and the display sheet 7, and the plurality of X electrodes and Y electrodes disposed in the flat panel input device 4 shields light emitted from the light source. Thus, there is a problem in that the display sheet 7 is likely to become dark.
In order to overcome this problem, if the X electrodes and the Y electrodes are formed using a transparent electrode (ITO), light emitted from the light source can pass through the ITO and thus brightly shines the characters, thereby solving the problem. There is, however, a problem in that the production cost is likely to rise because ITO is expensive.
Furthermore, if holes through which light emitted from the light source can directly pass are formed in the flat panel input device 4, the rear surface of the display sheet 7 can be brightly illuminated.
If the holes are formed in the flat panel input device 4, however, the X electrodes and the Y electrodes must be disposed to detour the positions of the holes while avoiding the positions of the holes. In this case, lines (X and Y electrodes) having the detours and lines (X and Y electrodes) not having the detours are mixed depending on formation positions of the holes. Due to this, there is a problem in that a coordinate position cannot be detected correctly because a capacitance between the respective lines is greatly different.
Furthermore, the capacitive coordinate detection device disclosed in Japanese Unexamined Patent Application Publication No. 08-137607 is constructed such that one of the plurality of X electrodes and Y electrodes is grounded (minus electrode), and the other of the plurality of X electrodes and Y electrodes is applied with a plus voltage. The X electrodes and the Y electrodes become minus electrodes or plus electrodes, if needed. Although wiring patterns having a plurality of leading lines for connection to other circuits are formed in the X electrodes and the Y electrodes, they must be inevitably disposed in a state where they are densely arranged in the vicinity of the X electrodes or the Y electrodes disposed outside as portable terminal apparatuses are miniaturized.
However, if the wiring patterns are densely disposed in the vicinity of the X electrodes or the Y electrodes disposed outside, a capacitance for the X electrodes or the Y electrodes becomes unstable. Accordingly, there is also a problem in that it is difficult to correctly detect coordinate positions.